Trophic selectivity has major influences on consumers’ fitness, stability of predator and prey populations, and nutrient fluxes in food webs. Trophic selectivity occurs when the relative abundances of resources in a consumer's diet differ from their relative abundances in the environment. This discrepancy between resources abundance and use has been predicted to increase with the availability of resources in the environment. Trophic selectivity has also been predicted to increase with the heterogeneity of resources quality in the environment. Despite their ecological and evolutionary implications, conclusive in situ tests of these predictions are still lacking. We challenged these two predictions by studying 15 closely related species of isopods distributed along a wide range of resource availability (from 1.3 to 57.6 grams of organic carbon per square meter). The dataset ranged from deep cave systems, considered as some of the most resource‐limited environments on Earth, to highly‐productive surface streams. For each species, we quantified the relative abundance of all available resources in the environment and estimated the heterogeneity in the stoichiometric quality of these resources. Isopod diet was determined using C and N stable isotopes and a Bayesian mixing model. The degree of trophic selectivity was then calculated for each species. By coupling a standardized quantification of trophic resources with a fine determination of diets using stable isotopes, we uncovered a positive relationship between resource availability and trophic selectivity. Contrary to our second prediction, trophic selectivity did not correlate with the heterogeneity of resources quality. Our results have important implications in trophic ecology by highlighting that resource availability was a main driver of trophic choices in aquatic invertebrates across a broad range of environments. These findings call for further evaluation of the mechanisms (e.g. trophic competition) causing the positive relationship between trophic selectivity and resource availability, as these mechanisms could be closely linked to those generating the documented relationship between species richness and productive energy.

1.Individuals aim to maximize their fitness by matching their own phenotype to the optimum phenotype in their environment. Individuals can achieve matching through several mechanisms including habitat choice and adaptive plasticity.

Senescence—the decline in age‐specific contribution to fitness with increasing age—has been widely investigated in evolutionary ecology. A tremendous amount of detailed empirical analyses have now revealed the widespread occurrence of demographic senescence (i.e. both actuarial and reproductive senescence) and have started to identify factors (e.g. environmental conditions) that modulate its timing and intensity, both within and across species. In this special feature, we have built on this flourishing work to highlight several axes of research that would benefit from more integrative and multidisciplinary approaches. Several contributions compiled in this special feature emphasize that our understanding of senescence remains taxonomically limited, mostly focused on birds and mammals, and is therefore not representative of the biological diversity displayed across the tree of life. In line with this observation, the influence of some peculiar lifestyles (e.g. involving sociality or modularity) on the evolution of senescence is yet to be deciphered. Understanding of the diversity of senescence patterns across and within species and among traits will necessitate the establishment of new metrics as a golden standard to fully account for age‐specific changes recorded in individuals’ performance. This is illustrated with the specific case of actuarial senescence. This special feature also highlights that the diversity of biological samples collected from wild plants and animals, along with accurate demographic data, is expanding. The fast development of new molecular tools now offers a unique opportunity to launch research programmes at the interface of physiology, health and ageing in non‐model organisms. We argue that while these different research axes constitute key avenues of investigations for the coming years, they are only the tip of the iceberg. To appreciate the full complexity of the senescence process in nature, from its evolutionary causes to its demographic consequences, we also need a better understanding of the role played by both environmental conditions and gene–environment interactions, of constraints, and of senescence, an improved assessment of the influence of individual heterogeneity, and the consideration of transgenerational effects when quantifying the fitness consequences of senescence.

Deciphering the mechanisms that drive variation in biomass production across plant communities of contrasting species composition and diversity is a main challenge of biodiversity–ecosystem functioning research. Niche complementarity and selection effect have been widely investigated to address biodiversity–productivity relationships. However, the overlooking of the specific role played by key species has limited so far our capacity to comprehensively assess the relative importance of other potential drivers of biodiversity effects. Here, we conducted a grassland diversity–productivity experiment to test how four potential facets of biodiversity effects, namely species richness, functional diversity, species identity and the relaxation of intraspecific competition, account for variations in above and root biomass production. We grew six plant species in monoculture, as well as in every combination of two, three and six species. Plant density was kept constant across the richness gradient but we additionally grew each species in half‐density monoculture to estimate the strength of intraspecific competition for each studied species. We characterized eight functional traits, including root traits, related to nutrient and light acquisition and computed both the functional dissimilarity and the community‐weighted mean (CWM) of each trait. We further partitioned above‐ground biodiversity effect into complementarity and selection effects. We observed strong positive biodiversity effects on both above‐ground and root biomass as well as strong positive complementarity effect. These arose largely from the presence of a particular species (Plantago lanceolata) and from CWM trait values more than from a higher functional dissimilarity in plant mixtures. P. lanceolata displayed the highest intraspecific competition, which was strongly relaxed in species mixtures. By contrast, the presence of Sanguisorba minor negatively affected the productivity of plant mixtures, this species suffering more from interspecific than intraspecific competition. This study provides strong evidences that the search for key species is critical to understand the role of species diversity on ecosystem functioning and demonstrates the major role that the balance between intraspecific and interspecific competition plays in biodiversity–ecosystem functioning relationships. Developing more integrative approaches in community and ecosystem ecology can offer opportunities to better understand the role that species diversity plays on ecosystem functioning.

1. The field metabolic rate (FMR) of an endothermic animal represents its energy expenditure in a natural environment, or its energy budget, and its field water flux (FWF) reflects the animal's water requirements.

1. Ecological specialisation is an important mechanism enhancing species coexistence within a given community. Yet, unravelling the effect of multiple selective evolutionary and ecological factors leading the process of specialisation remains a key challenge in ecology. Subterranean habitats provide highly replicated experimental arenas in which to disentangle the relative contribution of evolutionary history (convergent evolution vs. character displacement) and ecological setting (environmental filtering vs. competitive exclusion) in driving community assembly.

1.By providing food, living space and reducing biotic and abiotic stress, habitat‐formers support diverse communities. The characteristics that mediate their value to associated organisms, however, is not always clear. Traditional assessments of habitat‐former – associated community relationships use broad‐scale classifications to group habitat‐formers according to identity or gross morphology. Trait‐based approaches instead measure traits at the level of the individual and in doing so allow for predictions that are not constrained to taxonomic groups. By measuring at the level of the individual, trait‐based approaches have to potential to capture not only interspecific variability, but also the intraspecific variation that is often ignored by traditional approaches. Trait‐based approaches may therefore provide a novel way to link the characteristics of habitat‐formers to ecosystem functioning.

1.The life cycles of animals vary in relation to local climate, as a result of both direct environmental effects and population‐level variation in plastic responses. Insects often respond to the approach of winter by entering diapause, a hormonally programmed resting state where development is suspended and metabolism suppressed. Populations often differ in the duration of diapause, but the adaptive reasons for this are unclear.

Human influence on the environment is so extensive that virtually all ecosystems on the planet are now affected by biological invasions. And, often, ecosystems are invaded by multiple co‐occurring non‐native species. Hence, it is important to understand the impacts these invasions are producing on biodiversity and ecosystem processes. Here, we present results of a two‐year long field experiment where we tested the effects of co‐occurring invasive C4 African grasses in a Cerrado area in central Brazil. We compared plant and arthropod communities, plant biomass, and soil nitrogen dynamics and soil chemical characteristics across five experimental treatments: Urochloa decumbens removal, Melinis minutiflora removal, both U. decumbens, and M. minutiflora removal, U. decumbens and M. minutiflora invaded plots, and uninvaded Cerrado. We hypothesized that selective removal of invasive grasses would have distinct effects on the native ecosystem structure and functioning. We expected that each invasive grass would produce a different type of impact on the native ecosystem and that their impacts would be synergistic when co‐occurring. Removal of M. minutiflora doubled native plant diversity and biomass when compared to invaded plots, whereas removal of U. decumbens did not alter these parameters. Cerrado plots had four times more plant species than plots cleared of invasives. Removal of invasive grasses did not affect the species richness or community composition of soil epigeal fauna. Cerrado soils had lower fertility, organic matter content, and pH than invaded soils. The effects were generally higher when both invasive grasses were removed, suggesting impacts were synergistic, but M. minutiflora had greater effects on plants and soils than U. decumbens. Both invasive species produced negative impacts, but a single species was the main driver. We also detected persistent effects of the invasive grass species on the ecosystem after two years of removal. We conclude that invasive species of the same functional group have similar types of effects in native ecosystems, but the magnitude of impact was largely dependent on invasive species biomass and cover. Where multiple invasive species are present, research and management of invaded ecosystems should tackle the interacting effects of co‐occurring invaders.

Turgor loss point (πtlp) has been suggested to be a key trait for drought resistance in woody species. In herbaceous grassland species the role of πtlp for species drought survival has not yet been tested, although grasslands are projected to experience more frequent and intense droughts with climate change. To gain insights into the role of πtlp for drought resistance of temperate perennial grassland species, we assessed πtlp of 41 species common in Germany (20 forbs, 21 grasses). We directly related them to the species’ comparative whole‐plant drought survival and midday leaf water potentials under drought (ΨMD) assessed in a common garden drought experiment, and to species moisture association. Species drought survival increased with increasing πtlp across all species as well as within forbs or grasses separately. ΨMD was positively related to πtlp and drought survival. Our results imply that high πtlp promotes drought survival of common perennial European temperate mesic grassland species by enabling them to maintain high leaf water potentials under drought, i.e., a desiccation avoidance strategy. However, πtlp was not related to species moisture association. The positive relationship between πtlp and drought survival in herbaceous grassland species was opposite to the negative relationship previously established in woody plants, implying that mechanisms of drought resistance differ between woody and herbaceous species. Our results highlight the necessity of directly testing the relationship of functional traits to whole‐plant drought survival in different plant life forms, before using trait assessments for predicting plant responses to drought.

Biocrusts are critical components of drylands where they regulate a wide range of ecosystem functions, however, their response to the worldwide phenomenon of shrub encroachment and to livestock grazing, the most extensive land use in drylands, are not well studied. Grazing by livestock and increases in shrub cover could influence biocrust communities directly via trampling or shading, or indirectly, by altering biotic interactions amongst biocrust taxa. The extent of these changes in biocrust cover, diversity and composition are poorly known. We used linear models and structural equation modelling to examine the direct effects of grazing and shrubs on biocrust community composition and the indirect effects mediated by changes in species interactions. Biocrust richness and cover increased with increasing shrub cover at the site level. This pattern occurred despite the negative response we found (lower cover and richness) under shrub patches versus open areas, which was consistent irrespective of grazing level. Functional diversity and evenness were similar between shrubs and open at low grazing intensity, but at high grazing functional diversity was greater in the open. Competition between biocrust species was an important driver of their community assembly irrespective of shrub cover, grazing intensity or patch type. Structural equation models showed that the effects of grazing and shrub cover on functional evenness, functional diversity and richness were controlled by biotic interactions within the shrub microsites. In the open, however, these effects were either direct or mediated by changes in cover. Biocrust cover, species richness and functional diversity increase with shrub cover at the site scale, despite the negative effects at the microsite level. We demonstrate here that drivers of community assembly differ markedly at small spatial scales. Though biocrust communities were directly driven by environmental filtering in the open, biotic interactions played a fundamental role in their assembly when growing beneath shrubs.

The root economics spectrum (RES) hypothesis predicts that fast‐growing tree species have short‐lived roots with high specific root length (SRL) to allow rapid resource uptake, and opposite trait expressions for slow‐growing species. Yet, the mixed support for this hypothesis suggests that trees can adopt alternative strategies to increase resource uptake, besides an increase in SRL. We combined a novel mechanistic whole‐tree model and empirical fine‐root data of ten tree species to test the effects of one of these alternative strategies, notably increasing fine‐root mass, on the tree's net C gain (used here as a proxy for tree performance), and to assess how fine‐root lifespan influences the relative importance of SRL and fine‐root mass for the C balance of trees. Our results indicate that accounting for the short lifespan of high‐SRL roots has important implications for explaining tree performance and the role of roots herein. Without considering their faster turnover, high‐SRL roots and low fine‐root mass resulted in the highest performance as predicted from the RES. Yet, when their higher turnover rates were accounted for, a high fine‐root mass and low SRL lead to the highest performance. Both our model outcomes and field data further show a negative relationship between SRL and fine‐root mass through which species aim to realise a similar root length density. This trade‐off further indicates how high a SRL and low fine‐root mass as well as opposite trait values can both lead to a positive C balance in a similar environment. Our study may explain why high‐SRL roots do not necessarily lead to the fastest tree growth as often hypothesised and demonstrates the importance of fine‐root mass in combination with fine‐root lifespan for explaining interspecific differences in tree performance. More generally, our work demonstrates the value of identifying and investigating different belowground strategies across species from a whole‐plant modelling perspective, and identifies the relationship between SRL, fine‐root biomass and lifespan as an important functional dimension to variation in species’ performance.

1.Recent findings suggest that tree mortality and post‐drought recovery of gas exchange can be predicted from loss of function within the water transport system. Understanding the susceptibility of plants to hydraulic damage requires knowledge about the vulnerability of different plant organs to stress‐induced hydraulic dysfunction. This is particularly important in the context of vulnerability segmentation between plant tissues which is believed to protect more energetically “costly” tissues, such as woody stems, by sacrificing “cheaper” leaves early under drought conditions.

Most ectotherms follow the temperature‐size rule (TSR): in cold environments individuals grow slowly but reach a large asymptotic length. Intraspecific competition can induce plastic changes of growth rate and asymptotic length and competition may itself be modulated by temperature.

Morphological traits can influence the ability of animals to perform ecologically relevant tasks that are essential for their growth, survival, and fitness. Understanding such links in juvenile animals is of particular importance, as periods of juvenile life are often associated with high rates of mortality and can be critical with respect to evolution of life histories and viability of populations. To study such links, an important first step is to assess relationships between juvenile morphologies and measures of performance. Altricial songbirds can provide novel insights into morphology‐performance relationships owing to the high locomotor demand placed on juveniles after they leave the nest. Therefore, we examined the relationships between the size and developmental stage of juvenile traits and juvenile flight ability among 13 co‐existing songbirds in east‐central Illinois. Among potential traits, we focused on relationships between flight ability and four measures of wing development at fledging as well as two body measures thought to reflect an individual's overall quality and musculature. Among measures of wing development, wing emergence was the strongest, most consistent predictor of ability within and among species. Relative wing length and wing emergence‐loading were often good predictors of flight ability as well, while relative‐wing loading was rarely a good predictor. Estimates of body mass and body condition were poor, inconsistent predictors of flight ability both within and among species. Our results highlight the importance of wing development for flight performance of juvenile songbirds. Furthermore, our evaluation of different wing characteristics suggests that the importance of macro‐scale transmissivity and wing/disc loading toward flight ability changes with respect to juvenile ontogeny. Though mass likely interacts with wing characteristics in determining flight ability, our results suggest that body mass and condition are likely reflective of other, non‐flight measures of fledgling performance. Few studies on altricial songbirds have examined morphology‐performance relationships in juveniles. By linking wing emergence with juvenile flight ability, our research has taken a first step in assessing the adaptive significance of variation in wing development for juvenile songbirds.